Israel faces unprecedented pressure to abandon its official policy of “ambiguity” on its possession of nuclear weapons as the international community meets at the United Nations in New York this week to consider banning such arsenals from the Middle East.

Israel’s equivocal stance on its atomic status was shattered by reports on Monday that it offered to sell nuclear-armed Jericho missiles to South Africa’s apartheid regime back in 1975.

The revelations are deeply embarrassing to Israel given its long-standing opposition to signing the Nuclear Non-Proliferation Treaty, arguing instead that it is a “responsible power” that would never misuse nuclear weapons technologies if it acquired them.

Reports of Israel’s nuclear dealings with apartheid South Africa will also energise a draft proposal from Egypt to the UN non-proliferation review conference that Israel — as the only nuclear power in the region — be required to sign the treaty.

Israeli officials are already said to be discomfited by Washington’s decision earlier this month to agree a statement with other UN Security Council members calling for the establishment of a Middle East zone free of nuclear arms.

The policy is chiefly aimed at Iran, which is believed by the US and Israel to be secretly developing a nuclear bomb, but would also risk ensnaring Israel. The US has supported Israel’s ambiguity policy since the late 1960s.

Oversight of Israel’s programme is also due to be debated at a meeting of the UN’s nuclear watchdog, the International Atomic Energy Agency, in Vienna next month.

The administration of US President Barack Obama is reported to have held high-level discussions with Israel at the weekend to persuade it to consent to proposals for a 2012 conference to outlaw weapons of mass destruction in the Middle East.

As pressure mounts on Israel, local analysts have been debating the benefits of maintaining the ambiguity policy, with most warning that an erosion of the principle would lead inexorably to Israel being forced to dismantle its arsenal.

Echoing the Israeli security consensus, Yossi Melman, a military intelligence correspondent for the Haaretz newspaper, also cautioned that declaring Israel’s nuclear status “would play into Iran’s hands” by focusing attention on Tel Aviv rather than Tehran.

Israel refused to sign the 1970 Nuclear Non-Proliferation Treaty, having developed its first warhead a few years earlier with help from Britain and France.

Tom Segev, an Israeli historian, reported that Israel briefly considered showing its nuclear hand in 1967 when Shimon Peres, Israel’s current president, proposed publicly conducting a nuclear test to prevent the impending Six-Day War. However, the test was overruled by Levi Eshkol, the prime minister of the time.

Mr Peres, who master-minded the nuclear programme, later formulated the policy of ambiguity, in which Israel asserts only that it will “not be the first to introduce nuclear weapons to the Middle East”.

That stance — and a promise not to conduct nuclear tests — was accepted by the US administration of Richard Nixon in 1969.

According to analysts, the agreement between Israel and the US was driven in part by concerns that Washington would not be able to give Israel foreign aid — today worth billions of dollars — if Israel declared itself a nuclear state but refused international supervision.

Nonetheless, revelations over the years have made it increasingly difficult for the international community to turn a blind eye to Israel’s arsenal.

Mordechai Vanunu, a technician at the Dimona nuclear energy plant in the Negev, provided photographic evidence and detailed descriptions of the country’s weapons programme in 1986. Today the Israeli arsenal is estimated at more than 200 warheads.

In 2006 Ehud Olmert, then the prime minister, let slip Israel’s nuclear status during an interview with German TV when he listed “America, France, Israel and Russia” as countries with nuclear arms.

Even more damaging confirmation was provided this week by Britain’s Guardian newspaper, which published documents unearthed for a new book — The Unspoken Alliance by Sasha Polakow-Suransky, an American historian — on relations between Israel and South Africa’s apartheid regime.

The top-secret papers reveal that in 1975 Mr Peres, then Israel’s defence minister, met with his South African counterpart, P W Botha, to discuss selling the regime nuclear-armed missiles. The deal fell through partly because South Africa could not afford the weapons. Pretoria later developed its own bomb, almost certainly with Israel’s help.

Israel, Mr Polakow-Suransky said, had fought to prevent declassification of the documents.

Despite publication by the Guardian of a photographed agreement bearing the date and the signatures of both Mr Peres and Mr Botha, Mr Peres’ office issued a statement on Monday denying the report.

Israel’s increasingly transparent nuclear status is seen as an obstacle to US efforts both to impose sanctions on Iran and to damp down a wider potential nuclear arms race in the Middle East.

This month the US surprised officials in Tel Aviv by failing to keep Israel’s nuclear programme off the agenda of the IAEA’s next meeting, on June 7. The issue has only ever been discussed twice before, in 1988 and 1991.

Aware of the growing pressure of Israel to come clean, Benjamin Netanyahu, the Israeli prime minister, declined an invitation to attend a nuclear security conference in Washington last month at which participants had threatened to question Israel about its arms.

At the meeting, US President Barack Obama called on all countries, including Israel, to sign the Non-Proliferation Treaty.

A draft declaration being considered at the UN review conference later this week again demands that Israel — and two other states known to have nuclear weapons, India and Pakistan — sign the treaty.

Egypt has proposed that the 189 states that have signed the treaty, including the US, pledge not to transfer nuclear equipment, information, material or professional help to Israel until it does so.

Reuven Pedatzur, an Israeli defence analyst, warned recently in Haaretz that there was a danger the Egyptian proposal might be adopted by the US, or that it might be used as a stick to browbeat a recalcitrant Israel into accepting greater limitations on its arsenal. He suggested ending what he called the “ridiculous fiction” of the ambiguity policy.

Emily Landau, an arms control expert at Tel Aviv University, however, said that those who believed Israel should be more transparent were “misguided”. Ending ambiguity, she said, would eventually lead to calls for Israel’s “total and complete disarmament”.

The last Non-Proliferation Treaty review conference, five years ago, failed when the US repudiated pledges to disarm and refused to pressure Israel over its nuclear programme.